This invention relates to method and apparatus for pulling wire cables and fiber optic cables through underground conduit, and more particularly to pull ropes for pulling such cables.
Fiber optic cable is composed of a bundle of long, thin fibers of glass, plastic or other transparent material, closed within a protective sheath. Encoded light pulses carrying audio and video signals are sent through the fiber much like electric current travels along a wire. The advantage of fiber optic cable over conventional cable lies in its transmission characteristics. Because of the fiber's thinness and superior attenuation characteristics, a fiber optic cable can carry a much higher rate of information over many more channels than a comparably sized wire cable.
However, fiber optic cable is more difficult to lay than conventional cable. It lacks the tensional strength of conventional wire cable and will fracture at a much lower pulling tension. Furthermore, because of its construction, fiber optic cable is relatively inflexible. Typically, the fibers are bundled in a spiral fashion around a stiff steel support wire within a hard plastic protective sheath. Bending of the cable beyond a limited range can break the fibers within.
Because of these material drawbacks, conventional pulling methods and apparatus have proven inadequate for pulling more than a relatively short length of fiber optic cable through an underground conduit. These methods usually comprise placing a single winch at the conduit exit, passing a pull rope attached to the fiber optic cable through the conduit to the winch, and operating the winch to pull the rope and cable through the conduit until the rope is completely wound on the winch and the cable reaches the conduit exit. Pulling a cable in such a manner requires considerable tension to overcome the frictional drag of the cable and pull rope along the conduit surface. The winch pulling tension necessary to overcome this drag quickly increases as the length of pull increases. Lubricant is conventionally used to reduce drag but does not solve the problem. Typically, no more than 2,000 feet of cable can be pulled before the winch tension exceeds the cable's tensional limit. (However, the length of pull varies with the condition of the conduit.) In contrast, many times that length of wire cable can be pulled without the cable breaking.
The extra pulling requires more time and manpower in moving the winches and setting up the apparatus. Moreover, connecting the relatively short lengths of fiber optic cable adds substantial additional cost to installation of the cable. Each connection demands expensive and time-consuming splicing. The extra splicing in turn creates resistance to the transmitted light pulses which must be overcome by the installation of additional signal repeaters along the cable to boost the signal strength.
To increase the maximum continuous length of fiber optic cable which may be pulled, several techniques have been developed. My own prior invention of a intermediate capstan winch system and pulling method, described in my above-cited patents has proven to be the most effective over nearly five years of use. Nevertheless, improvements remain to be made.
Typically, pulling cable through conduit by conventional means begins by connecting the cable to be pulled to a pull rope that has previously been placed in a conduit. The preferred method for installing the pull rope is to blow it in to the conduit pneumatically. The pull rope is then connected to the cable at the entry end of the conduit and is pulled toward the exit end of the conduit by a winch located at the exit end. Usually, the cable comes wound on a spool, and as the cable is pulled through the entry end of the conduit, the cable unwinds from the spool.
Frequently, conduit for cable is placed in the same passageway along with other underground utilities through a network of manholes. Because space for these utilities is limited, conduit must often be routed around other utilities by curving and twisting the conduit. Moreover, conduit must sometimes be installed in places where it is necessary to turn at 90 degree angles, e.g., when cables are routed into buildings from underground manholes.
Each place where the conduit bends, the outer surfaces of the pull rope and cable chafe against the inside of the conduit as each are pulled through the conduit. As a result, friction and resistance are generated. In addition, even in straight conduit, there can be substantial resistance and friction between the pull rope or cable against the conduit.
As a result of friction and resistance, tension is created in the pull rope. Since friction occurs over the whole length of pull rope, the largest tensile forces will occur at the most downstream portion of the conduit, i.e., at the exit end of the conduit. Consequently, this causes the highest levels of abrasion, on the conduit and on the rope, to occur toward the exit end of the conduit, on long pulls of cable.
Since lengths of conduit are commonly 2000 feet and longer, and the length of pull rope in successive segments of conduit is sometimes longer than 10,000 feet, extensive friction by the pull rope against the conduit is generated. Accordingly, the friction generated by longer pulls can cause extensive abrasion of the pull rope, the conduit, and the cable.
In addition, the amount stretch in the pull rope is another factor that a cable installer must consider. As the length of pull becomes longer and the tensile forces become greater, the pull rope will be subject to a large amount of stretch. The additional amount of stretch in longer pulls may become critical thus limiting the maximum length of pull.
In light of the foregoing, there is an established need for low friction pull ropes that are less inclined to abrade and stretch.
Today, the most common types of pull rope are made from fibers of either polyethylene, polyester, cotton, or a combination thereof. Typically, these ropes are either twisted or braided. In addition, ropes made with a braided plastic outer cover in combination with a polyester core have also been used for pulling cable. As will be seen, however, each of the above materials and rope structures have drawbacks that make the search for a more suitable rope a continuing exercise.
One of the most widely used materials used in producing pull ropes is polyethylene. Generally, polyethylene rope is either single braided or twisted, and is usually is yellow in color. Even though twisted Polyethylene is somewhat more common as a pulling rope, both the twisted and the single braided ropes share the same disadvantages. Included as disadvantages are stiffness which makes it difficult to blow into conduit, high elasticity manifested by excessive stretching; abrasiveness to the point of cutting PVC conduit; high resistance from friction when hot thus increasing tension forces. Finally, polyethylene rope will not hold water that is sometimes required for lubrication during the pull.
Polyester rope is softer and therefore more readily blown into conduit. Moreover, braided polyester rope is less prone to stretch than is polyethylene rope. Never the less, polyester rope has several disadvantages. The most notable is that it is highly resistive, it frays easily, and it wears out quickly.
Finally, a more recent addition to the assortment of pull ropes is rope constructed of a braided plastic outer cover over a braided or twisted polyester core. This type of pull rope has the advantage of low friction and thus low abrasion resulting in little need for lubrication. However, this type of rope also has disadvantages, e.g., it is difficult to splice, it is difficult to tie knots, it is hard to blow in conduit, it does not store easy, and it is very bulky.
In the pulling of fiber optic cable, experience has proven it desirable to divide a long conduit, e.g., 20,000 or more feet, into shorter, e.g., 2000 to 3000 foot lengths, and to place intermediate capstan winches at manholes or access openings at the end of each segment. This arrangement allows long, unbroken fiber optic cables to be pulled into conduit without exceeding the maximum tensile strength and bending radius limits of the cable. It means, however, that the pull rope must be pulled over very long distances over which it is subjected to high abrasion and wear. It also means that the conduit itself, particularly the downstream segments, are subjected to substantial abrasion from pulling tens of thousands of feet of continuous pull rope through it. This factor in pulling fiber optic cable makes the disadvantages of conventional pull ropes more critical in modern fiber optic cable placement methods.
In addition, the use of intermediate capstan winches requires that operators be able to control frictional engagement of the pull rope and cable surfaces with the circumferential surfaces of the winch. By using conventional pull ropes that create higher levels of friction and resistance on such surfaces, an operator could find that engagement and disengagement of the frictional surfaces are difficult to control. Consequently, this could result in too much tension, which in turn could damage the fiber optic cable.
Another consideration is that conventional pull ropes have considerably more frictional resistance against conduit inner surfaces than do fiber optic cables. Because of this, the transition from pull rope to fiber optic cable over an intermediate capstan could be difficult to control by the operator. One way to lessen the effects of this transition would be to use a low friction pull rope, i.e., one with a frictional resistance closer to that of fiber optic cable.
For all of the foregoing reasons, the need remains for a method and apparatus for pulling long runs of fiber optic cable quickly and efficiently while keeping friction and resistance to a minimum.